Galvanic corrosion is an electrochemical process in which one metal corrodes preferentially to another metal when both metals are in electrical contact and exposed to a continuous electrolyte. Such a configuration is often referred to as a “galvanic couple,” and results when each metal has a different electrode potential. Various metals are often ranked according to their electrode potential in the “Anodic Index,” with magnesium having a generally more negative potential than steel, steel having a generally more negative potential than copper, and gold having the noblest potential.
In a galvanic couple, the cathode anodically polarizes the anode and accelerates the dissolution of the anode in the electrolyte. This leads to the anodic metal corroding more quickly than it otherwise would, while the corrosion of the cathodic metal is retarded even to the point of stopping.